The invention herein described relates generally to magnetic resonance imaging (MRI). It finds particular applications in imaging "time-varying" objects such as the beating heart or abdomen. However, the invention is also applicable to other imaging modalities including X-ray CT (computerized tomography), PET (positron emission tomography), and the like.
In 1973 Dr. Paul C. Lauterbur introduced to the public the first MR generated image. Since then, great strides have been made in MR imaging techniques and machinery. In particular, MR imaging of selected parts of the human body is a favored and popular method used by doctors in medical diagnostic procedures. The advantages of MR imaging over other imaging modalities include high spatial resolution, non invasiveness, and the ease of both 2-dimensional (2D) and 3-dimensional (3D) imaging.
A continuing and unsolved problem with MR imaging is the presence of artifacts or "ghosts" caused by the motion of the human body. Body motion is generally described in two distinct ways: 1) as a periodic gross motion, which includes head and limb displacement, particularly where the motion is due to the more or less voluntary actions by the patient and cannot be determined a priori; and 2) physiological movement, which includes respiratory motion, cardiac pulsations, aortic pulsations, and gastrointestinal peristalsis, which is relatively periodic.
Ghosts are particularly apparent when the motion is periodic or nearly so, and is caused by the inconsistent appearance of the object from view to view. MR imaging techniques have evolved to such a point that the problems of motion artifacts are one of the few remaining limitations in MR technology. There currently exists no method to eliminate motion problems satisfactorily, particularly for generalized, a periodic motion. Accordingly, MR imaging is used extensively only in medical applications where the gross movement of the patient can be kept to a minimum. One example of such use is where a patient is given a brain scan. There, the patient is asked to keep his/her head still during data acquisition or the patient's head is immobilized using straps and other techniques which help minimize any motion of that part of the body being imaged. Where there is no motion, image artifacts do not present a problem. Thus, MR imaging is currently generally best used in obtaining images of the body where the patient's movement can be eliminated or restricted, thereby preventing or minimizing the appearance of ghosts and other motion artifacts which act to degrade the final image.
A particularly useful application of MR technology would be imaging of anatomical structures experiencing physiological motion, commonly manifested as periodic displacement due to respiration. Unfortunately, in this very important application of MR imaging, ghosting artifacts are especially problematic because, for example, typical abdominal images contain several moving anatomical structures, each generating ghosts with different shapes and positions. Such image artifacts make reading of the images difficult, and may possibly even lead to misdiagnosis.
In many imaging modalities, the measured signal S(k) is collected in the spatial-frequency domain (known as k-space), resulting in the imaging equation ##EQU1## where .rho.(r) is the desired image function. For stationary objects, k-space can be sufficiently sampled, and the image function .rho.(r) can be accurately recovered from the measured data by the discrete inverse Fourier transform. However, when the object is moving during data acquisition, as is the case with physiological motion, only a limited portion of k-space can be sampled instantaneously. If motion is ignored, as is often the case with conventional imaging methods, significant motion artifacts can arise. For example, for periodic or quasi-periodic motion, the motion effect manifests itself as image blurring and ghosting artifacts.
Over the past decade, several methods have been proposed to cope with motion artifacts. Such techniques have included, but have not been limited to, patient restraint, averaging, gating, ordered phase encoding, and various post-processing techniques. However, none of these attempts at reducing ghosting artifacts have fully succeeded.
Patient restraint, the simplest of all the solutions to implement is unfortunately also one of the least convenient for the patient. This method involves controlling the respiratory motion of patients by either asking them to hold their breaths, or by strapping the chest down to minimize the expansion of the chest or abdomen. It is obvious that such methods require absolute cooperation from the patient, which cooperation may not always be feasible or forthcoming. Additionally, even though chest motion may be preventable, other internal organs, such as the heart, obviously cannot be stilled.
Averaging, which is the process of averaging data acquired under conditions that are identical except for the motion, has the effect of attenuating the ghost artifacts and reducing random noise. In particular, the phase has the greatest significance for averaging. In fact, it is when the motion during each data set is offset in phase that the motion artifacts are attenuated. However, for averaging to be effective, a large number of data sets are required. In actual practice, up to eighteen data sets may be used in the averaging process. This results in a slowing down of the imaging process. Another disadvantage of this technique is that blurring caused by motion is not reduced.
Respiration normally imparts a spatial frequency dependence to the position of an anatomical structure. That is to say, respiration causes the subject to periodically vary in position. Respiratory gating is a method whereby image acquisition occurs only when the subject of the imaging has the same configuration as it had at a previous time. An inherent disadvantage of this method is that imaging times may take up to twice as long as normal.
Ordered phase encoding eliminates ghosts by acquiring data for periodic motion during one half or one period of the motion. This is accomplished by monitoring the motion, and from the displacement determining which views to acquire. Thus, after the views have been acquired, they are ordered so as to make motion appear monotonic. Although this method removes many of the ghosts without increasing the imaging time, it does not remove the blurring.
One of the most significant advances in motion compensation techniques is the use of navigator (NAV) echoes for estimating translational displacement parameters as described by R. L. Ehman and J. F. Felmlee in their article "Adaptive Technique for High-Definition MR Imaging of Moving Structures," Radiology, vol. 173, no. 1 pp. 255-63, October 1989. Generally, the NAV echo is similar to an image echo, except that no phase encoding or a small amount of constant phase encoding is applied thus its one-dimensional Fourier transform gives a projection throughout the navigator directions (NAV projections). NAV echoes are designed to permit tracking of the position of an object of interest within the field of view. Before or after each view, a NAV echo in either the frequency encoding or phase-encoding direction is acquired. A single NAV projection is then arbitrarily chosen as a reference, and cross-correlation functions are determined with respect to each of the other navigation projections. The peak location in the cross-correlation functions gives the relative displacement along the navigator direction. The disadvantage inherent in this technique is that only the in-plane bulk rigid-body motion along the navigator direction can be detected and subsequently removed. This is normally not the case in practical physiological motion, where the motion may be localized, non-rigid-body type and is arbitrary in direction.
Therefore, in spite of these advances in MR technology, imaging of time-varying objects remains a most challenging problem.